Mn—Mo—Ni steel has been known to have excellent strength and toughness, and is mainly used as a material for, for example, a pressure vessel of a nuclear power plant. Such a material has been required to have a toughness level that is increasingly raised from the viewpoint of safety. For example, higher low-temperature toughness is required for a cask used for storage/transport of spent fuel from a nuclear power plant. In addition, higher drop-weight characteristics at low temperature are required for the cask to ensure safety against failure. In step with these, Mn—Mo—Ni-series welding metals used for such applications are also required to be further improved in strength, low-temperature toughness, and drop-weight characteristics.
A welded structure including the Mn—Mo—Ni-series welding metal (an Mn—Mo—Ni-series welded structure) is subjected to long annealing for stress relief after welding (hereinafter, referred to as SR annealing), and carbide particles are precipitated during the SR annealing, causing variations in characteristics of the welding metal. Hence, there is a need of establishment of a technique for improving strength, low-temperature toughness, and drop-weight characteristics depending on conditions of SR annealing.
For example, Ni-based-alloy welding materials as disclosed in Patent Literature 1 and 9%-Ni-based-alloy welding materials as disclosed in Patent Literature 2 are known to be effective for improvement in low-temperature toughness of a welding metal. However, the Ni-based-alloy welding materials are disadvantageous in cost since the materials contain a large amount of expensive Ni. In addition, the 9%-Ni-based-alloy welding materials each have a stable austenite structure formed during SR annealing, causing a significant reduction in yield stress. Hence, there is a need of a technique that further improves strength, low-temperature toughness, and drop-weight characteristics of the welding metal while controlling the Ni content at a low level.
High toughness is expected in TIG welding having a low oxygen content. However, the TIG welding has a disadvantage of low construction efficiency. Hence, industrially, a technique ensuring high strength, low-temperature toughness, and drop-weight characteristics is desired in high efficient welding construction such as submerge arc welding having a high oxygen content.
On the other hand, for example, Patent Literature 3 discloses a certain effect of improving low-temperature toughness of a welding metal through formation of a fine acicular-ferrite structure nucleating on Ti-based oxide. In this technique, however, the lowest temperature at which sufficient low-temperature toughness is obtained is still not so low, −60° C. If a larger amount of Ti-based oxide is dispersed for further improvement in low-temperature toughness, coarse Ti oxide, which acts as origin of the fracture, increases. Hence, further devising is required. Patent Literature 4 discloses a method for achieving a welding metal having excellent drop-weight characteristics through controlling flux components and wire components in submerge arc welding. This technique, however, does not assume SR annealing. As a result, the lowest no-break performance temperature of the drop-weight characteristics is still not so low, −90° C. Furthermore, a welding metal having excellent toughness is obtained through controlling the Ni content as disclosed in Patent Literature 5 and welding metal having excellent toughness by using Ti as disclosed in Patent Literature 6. However, similarly to Patent Literature 4, both Literatures do not assume SR annealing.
As a technique in consideration of toughness after SR annealing, a technique disclosed in Patent Literature 7 is proposed. A level of toughness obtained after the SR annealing is about an absorbed energy of 55 J at −75° C. in the best case. This toughness still needs further improvement. Furthermore, effect of this technique to drop-weight characteristics is unknown. Patent Literature 8 discloses a technique for improving toughness of a high strength MIG welding metal by controlling a wire composition and a shield gas component. The ensured temperature, however, is still not so low, −50° C. Furthermore, effect of this technique to drop-weight characteristics is unknown. Moreover, Patent Literature 9 discloses a technique for achieving high strength and toughness by applying thermal treatment of quenching and tempering to a welding metal. However, application of quenching to the welding metal causes extremely cumbersome process.